Iowa Code § 356.16: Hard labor.

Able-bodied persons over the age of sixteen, confined in any jail under the judgment of any tribunal authorized to imprison for the violation of any law, ordinance, bylaw or police regulation, may be required to labor during the whole or part of the time of their sentences, as hereinafter provided, and such tribunal, when passing final judgment of imprisonment, whether for nonpayment of fine or otherwise, shall have the power to and shall determine whether such imprisonment shall be at hard labor or not.